[fanboy rant] I don't get the confusion over the shield generator on Hoth. The AT-ATs didn't have to contend with it because the shield exists ABOVE the Rebel base only, not at ground level. The starwars.com databank entry is pretty clear about it:
Ground-based installations use immense projectors to generate a deflector shield, like the one that protected the Rebel base on Hoth from orbital bombardment. This shield worked in both directions, however, and had to be lowered momentarily to allow the launching of evacuation ships. During the Battle of Hoth, Imperial ground craft landed beyond the shield perimeter, and marched over land to destroy the generators powering the shield.
Given this, I think any addition of a field effect for the walkers to go through would be not only unecessary, but just plain a bad idea. THERE'S NO SHIELD AT GROUND LEVEL. [/fanboy rant]